The invention applies to the field of equipment for tyre repair specialists and in particular to that of the wheel service machines.
It should be noted that the term wheel (for vehicle) means the coupling between a tyre and a corresponding rim (that is, the overall tyre/rim).
Wheel service machines are divided into two main types:                balancing machines, configured to measure the static and/or dynamic unbalancing of a wheel;        machines for fitting and removing a tyre on a corresponding rim (also known in the prior art as a “tyre changer machine”).        
The balancing of a wheel for a vehicle is performed by the tyre repair specialist in order to eliminate or reduce to a minimum the effects of the asymmetric distribution of the weights of the tyre/rim assembly. These asymmetries are usually compensated for by the tyre repair specialist by fixing counterweights to the rim.
The asymmetry is caused by the non perfect roundness of the tyre or of the rim. This is due basically to machining defects, non-homogeneity of construction materials, and the existence of inflation valves which, although relatively light in weight, constitute additional unbalanced weights. Further causes of the wheel unbalance are linked, for example, to deformation of the rims (caused by impacts and deep potholes), out of roundness of the tyre after a certain mileage, flattening of the tread during sharp braking, and errors during fitting and removal of the wheel.
These unbalances generate forces which cause annoying vibrations at the steering wheel, chassis and frame of the vehicle, in particular at high speeds.
The forces are called, depending on the direction in which act:                radial forces (in a direction which connects the centre of the rim to the tyre tread);        lateral forces (in the direction of the axis of rotation of the wheel);        tangential forces (in a direction which is tangential to the tread).        
The asymmetries and non-uniformity of the rim/tyre assembly cause variations to the radial, lateral and tangential force during rotation of the wheel subjected to a load.
Balancing machines comprise a frame from which a motorised spindle protrudes. A locking system makes it possible to fix the rim to the spindle, in such a way that the wheel can be placed in rotation in order to perform the measurements needed for the balancing operations. These machines measure the wheel unbalance by analysing the timing and amplitude of the mechanical vibrations which are generated by rotating the wheel. The mechanical vibrations are measured in terms of movements, forces or pressures, by using transducers which convert the measurements collected into electrical signals. Balancing machines indicate to the user the weight and the position on the rim at which to fix the counterweights.
Machines for fitting and removing a tyre on a rim comprise a wheel-holder unit and at least one operating unit. The wheel-holder unit usually comprises a locking system which fixes the rim which fixed to a rotary supporting plate, to set it in rotation about its axis of rotation. The operating unit is usually equipped with numerous tools, used for bead breaking, removing and fitting the tyre.
During the removal, the wheel is fixed to the wheel-holder unit and set in rotation whilst a removal tool, located in the stationary position relative to the movement of the rim, is operated so as to grip a portion of a bead of the tyre and extract it from the seat of the rim. The seat of the rim is formed by the zone between the flanges (that is, annular edges) of the rim.
During the fitting, the rim is fixed to the wheel-holder unit and set in rotation whilst a fitting tool, located in the stationary position relative to the movement of the rim, is operated so as to force a zone of a bead of the tyre inside the seat of the rim.
The two types of wheel service machines, that is, balancing and tyre changer, have different features and requirements.
In the balancing machines, the wheel is rotated at an angular speed greater than that of the tyre changer machines.
In order to measure the unbalances and the forces with a high degree of accuracy, the balancing machines require a system for locking the rim which is particularly precise and stable. More specifically, the system for locking the rim must be able to centre the rim extremely precisely and maintain this precision during rotation of the wheel.
In the tyre changer machines, the speed of rotation of the rim is less, but the action of the tools of the operating unit on the tyre generates high forces, which have an effect on the wheel-holder unit and on the frame of the machine. For this reason, the wheel-holder unit and the locking system of the tyre changer machines must be particularly robust and reliable.
There are also prior art wheel service machines equipped with a load roller, which, positioned at the periphery of the tyre, simulates a load applied to the tread during rotation of the wheel.
Examples of balancing machines equipped with a load roller are described in patent documents U.S. Pat. No. 8,250,915B1 and U.S. Pat. No. 6,405,591B1. Examples of tyre changer machines equipped with a load roller are described in patent documents U.S. Pat. No. 8,250,915B1, WO2014/129476A1, EP2468541A1, CN101298231A and EP2361791B1.
The use of the roller in a wheel service machine has the advantage of providing information useful to the tyre repair specialist, but there are various issues. In fact, the tyre service specialist with useful information. In effect, even if the vehicle wheel is balanced by counterweights, some non-uniformity in the tyre structure may give rise to lateral forces when the wheel is set in rotation under the action of a load
One issue concerns the fixing or locking of the wheel on the rotary wheel-holder unit, in particular in the case of tyre changer machines. The locking system used in the tyre changer machines is robust, but it is a relatively imprecise. This limits the accuracy of the measurements performed with the roller, thus adversely affecting the reliability.
Another issue concerns the overall dimensions, as the roller tends be bulky. That issue is particularly felt in the case of tyre changer machines equipped with a roller, as the tyre changer machines are equipped with numerous tools, which must be used by the tyre repair specialist on the wheel, simultaneously with or at different stages of the use of the tyre changer machine (for example, bead breakers, removal, fitting). In light of this, it should be noted that it also important that the tyre repair specialist has a large free space for moving around the wheel.
Thus, a tyre changer machine has an operating unit comprising a plurality of tools, to which the roller is added. For this reason, it is necessary to avoid interference between the tools of the operating unit and the roller itself. It is also necessary to allow the tyre repair specialist to alternately use the load roller and the tools of the operating unit in a reliable and fast manner, without losing time; it is also necessary to provide the tyre repair specialist with operating space which is as large as possible.
To improve the characteristics of the tyre and reduce its non-uniformity, the tyre service specialist usually removes parts of the tyre from the tread. In actual fact, this solution constitutes only a temporary remedy which often proves unsatisfactory because the amplitude of the vibrations produced remains high. Further, removing parts of the tyre from the tread means reducing the thickness of the tread and, consequently, shortening the working life of the tyre.
Among the causes of lateral forces connected with non-uniformity of tyre structure are defects known as conicity and ply steer.
To better understand the concept of conicity, let us imagine that during rotation, a wheel subjected to a load adopts a frusto-conical shape (that is, a first side wall of the tyre is larger in diameter than a second side wall. Consequently, it generates a force directed towards the apex of the cone from the first side wall to the second side wall. It should be noted that this force does not change direction if the direction of rotation of the wheel is reversed. By definition,
  Conicità  =                    FLT        cw            +              FLT        ccw              2  where FLTcw denotes a total lateral force measured in a first direction of rotation and FLTccw denotes a total lateral force measured in a second direction of rotation opposite to the first.
Conicity is generally associated with non-uniformity in the tyre structure such that one side wall is more rigid than the other.
Ply steer generates lateral forces which can cause the vehicle to deviate from a straight direction of travel. These forces are generated by non-uniformity in the distribution of the outer ply layers of the tyre. These forces change direction if the direction of rotation of the wheel is reversed. By definition,
  Plysteer  =                    FLT        cw            -              FLT        ccw              2  where FLTcw denotes a total lateral force measured in a first direction of rotation and FLTccw denotes a total lateral force measured in a second direction of rotation opposite to the first.
Further information useful for the tyre service specialist can be obtained by measuring the rolling radius. On account of tyre flattening, the rolling radius of the wheel (that is, the distance between the axis of rotation of the wheel and the point of contact between tyre and load roller) is smaller than the nominal radius of the wheel (that is, the radius of the wheel when not subjected to a load). The industry reference standards (for example, ETRTO Standards Manual) define the theoretical rolling circumference, that is, the value of the dynamic circumference theoretically adopted by the wheel when subjected to a maximum predetermined load, at a speed of rotation of 60 km/h and a reference inflation pressure, as follows.Cdin_lim=3.05dn where dn is the nominal diameter of the wheel (that is, the diameter of the wheel not subjected to the action of a load). To verify these standards, it is therefore necessary to set the wheel in rotation at a relatively high speed which wheel service machines, especially tyre changer machines, are not capable of reaching.
Another issue relates in general to all the wheel service machines equipped with a roller, and it relates to the completeness and the actual utility for diagnostic purposes of the data provided to the machine thanks to the measurements of the roller.
The wheel service machines equipped with a load roller provide data relating to a variation in the radial force (Radial Force Variation, RFV) and tangential force (Lateral Force Variation, LFV) during rotation of the wheel; moreover, a further parameter which may be measured by the wheel service machines equipped with a load roller is the conicity (by rotating the tyre in both directions), which depends on the behaviour of the lateral force.
However, these measurements do not allow defects to be distinguished which are linked to the conicity, or, more generally, the elastic response of the wheel, from defects which are linked to the structure of the tyre (due, for example, by possible damage). Moreover, these measurements do not provide diagnostic information on the noise level of the tyre.